Digital data file encryption apparatus and method and recording medium for recording digital data file encryption program thereon

ABSTRACT

A digital data file encryption system and method where digital data encrypted to a high level and an encryption key are received and stored in a computer. The stored, encrypted digital data is decrypted on the basis of the encryption key and stored in a buffer of a predetermined size. Output digital data from the buffer is reencrypted to a low level of encryption and downloaded to a digital data playing device or a data storage medium. Therefore, a digital audio or video data file is prevented from being diverted in unencrypted form from a computer communication network when it is transmitted to a personal computer through the computer communication network and, in turn, downloaded to the digital data playing device. Further, the digital data playing device is prevented from rising in cost due to the use of simple encryption for data downloaded to the playing device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a reissue application of application Ser. No.09/527,670 filed on Mar. 17, 2000 (now U.S. Pat. No. 7,130,246 ). Thepresent application also claims priority to Korean Patent ApplicationNo. 1999/9014 filed in the Republic of Korea on Mar. 17, 1999, which isincorporated by reference it its entirety. There are multiple reissueapplications related to U.S. Pat. No. 7,130,246 including Ser. No.12/252,295 filed Oct. 15, 2008; Ser. No. 12/252,300 filed Oct. 15, 2008(the present application); Ser. No. 12/252,306 filed Oct. 15, 2008; andSer. No. 12/252,311 filed Oct. 15, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to digital data file encryption in whichan encrypted digital data file is prevented from illicit interceptionand decoding when digital audio data or digital video data istransmitted to a personal computer through a computer communicationnetwork and, in turn, downloaded to a digital data player such as an MP3player.

2. Description of the Related Art

MP3 (shorthand for MPEG-1 Layer 3 Audio) is one of various availabledigital data formats for audio data. An MP3 player is a new notional,portable digital device capable of readily downloading and reproducingdesired data from a computer communication network using an audio datacompression coding technique prescribed in MPEG-1 Layer 3. The MP3player has few faults and excellent sound quality because it stores afile in the form of digital data. Further, the MP3 player is small insize and light in weight, thereby assuring high portability such that auser can carry it even during his physical exercise. For these reasons,this product is a viable alternative to a portable cassette taperecorder and compact disk (CD) player.

With reference to FIG. 1, there is shown in block form a conventionalarrangement of a digital data player and associated peripheral devices.In this drawing, the reference numeral 10 denotes a digital data serverwhich assigns an identification (ID) number and password (PWD) to apersonal computer 20 for user registration, and which also transmits adigital data player 22 in software form to the personal computer 20.Upon receiving a file supply request from the user, the digital dataserver 10 identifies the user on the basis of an ID number and passwordentered by him and supplies an encrypted digital data file to the userin accordance with this identification. The personal computer 20 storesthe digital data file supplied from the digital data server 10 on a harddisk 21 therein and decrypts it through the downloaded software player22 to reproduce the resultant unencrypted digital data file or todownload it to a digital data playing device 30. The digital dataplaying device 30 downloads the unencrypted data file from the personalcomputer 20 and stores it in a memory unit 40 for the reproductionthereof. The memory unit 40 downloads the unencrypted digital data filefrom the digital data playing device 30 and stores it in its internalmemory 42 to output the file for a desired playing operation.

The operation of the conventional arrangement with the above-mentionedconstruction will now be described.

In order to legally receive a desired digital data file from the digitaldata server 10, the user has to register with a digital data filesupplier. During user registration, the user is assigned an ID numberand password from the digital data file supplier. Then, the userdownloads a digital data player 22 in software form from the digitaldata server 10 through a communication network and installs thedownloaded digital data player 22 in the personal computer 20.

Thereafter, to download a desired digital data file from the digitaldata server 10 through the personal computer 20 and communicationnetwork, the user transmits his ID number and password to the digitaldata server 10 through the personal computer 20 and communicationnetwork. The digital data server 10 identifies the user on the basis ofthe transmitted ID number and password and supplies the desired digitaldata file to the user in accordance with the identification. At thistime, the digital data server 10 encrypts the digital data file usingthe user's ID number as an encryption key and transmits the encrypteddigital data file to the personal computer 20.

The personal computer 20 stores the digital data file transmitted fromthe digital data server 10 on the hard disk 21. Then, upon receiving areproduction request from the user, the personal computer 20 decryptsand reproduces the stored digital data file via the digital datasoftware player 22. As a result, the user is able to listen to desiredmusic through the personal computer 20.

On the other hand, if the user intends to listen to music in a digitaldata file form using the portable digital data playing device 30, thenthe personal computer 20 decrypts the digital data file, stored on thehard disk 21 with the digital data software player 22, and sends thedecrypted digital data file to the digital data playing device 30through a download unit 23 therein and the communication network.

Then, the digital data playing device 30 stores the digital data file,sent along the above path, in the memory 42 of the memory unit 40, whichis typically in the form of a removable card. If the user requests thedigital data player 30 to reproduce the digital data file stored in thememory 42, then the digital data player 30 reads the stored digital datafile from the memory 42 and reproduces it through a decoder 32 therein.As a result, the user can listen to desired music anywhere using thedigital data player 30.

However, the above-mentioned conventional arrangement has a disadvantagein that the digital data file may be intercepted from the communicationnetwork during downloading from the personal computer to the digitaldata playing device (or from the digital data playing device to thememory card) in an unencrypted condition. Such an interception of theunencrypted digital data file makes it impossible to protect thecopyright of a music copyright holder and music copyright associates(for example, a music producer and planner taking charge of musicproduction, duplication and distribution). In order to solve the aboveproblem, a conventional powerful encryption method may be used topowerfully encrypt the digital data file and send it to the digital dataplayer. However, such powerful encryption, in turn, requires a powerfuldecryption function, thereby increasing the cost of the digital dataplayer.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide anencryption system in which illegal outflow of a digital data file, suchas MP3, downloaded from a server can be prevented, with no additionalincrease in cost.

In accordance with one aspect of the present invention, there isprovided a method for encrypting digital data including decryptingdigital data which has been encrypted at a high encryption level,storing a predetermined amount of the decrypted digital data in abuffer, reencrypting output digital data from the buffer at a lowencryption level; and transferring the reencrypted digital data to adigital data player or a data storage medium.

In accordance with another aspect of the present invention, there isprovided a method for encrypting digital data including determiningwhether digital data which has been encrypted at a high encryption levelmust be protected from unauthorized copying, decrypting the digitaldata, transferring the decrypted digital data to a digital data playeror a data storage medium if the decrypted digital data need not beprotected from unauthorized copying, and reencrypting the decrypteddigital data at a low encryption level if the decrypted digital datamust be protected from unauthorized copying.

In accordance with a further aspect of the present invention, there isprovided a program (or script) embodied on a computer-readable mediumfor encrypting or decrypting a digital data file, thecomputer-readable-medium-embodied program including a first program codesegment to receive and store digital data encrypted to a high level andan encryption key, a second program code segment to decrypt the storeddigital data using the encryption key, a third program code segment tostore a predetermined amount of the decrypted digital data in a buffer,and a fourth program code segment to reencrypt the digital data from thebuffer to a low level and download the reencrypted digital data to adigital data player or a data storage medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram of a conventional arrangement of a digitaldata player and the associated peripheral devices

FIG. 2 is a block diagram of an embodiment of a digital data fileencryption apparatus in accordance with the present invention;

FIGS. 3A to 3E are views illustrating examples of file encryption inaccordance with the present invention;

FIG. 4 is a flowchart illustrating a digital data file encryption methodin accordance with the present invention;

FIG. 5 is a block diagram of an alternative embodiment of the digitaldata file encryption apparatus in accordance with the present invention;and

FIG. 6 is a diagram illustrating the digital data file encryption methodshown in FIG. 4, and the flow of the data file among the components ofthe digital data file encryption apparatus shown in FIGS. 2 and 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 2, there is shown in block form an embodiment ofa digital data file encryption system in accordance with the presentinvention. FIG. 6 illustrates the flow of the data file among thecomponents of the digital data file encryption system. The operation ofthe digital data file encryption system according to the presentinvention will hereinafter be described in detail with reference toFIGS. 2 to 4 and 6.

First, the user must register with a digital data file supplier tolegally receive a desired digital data file from a digital data server110 (FIG. 6, Step 400). During user registration, the user is assignedan ID number and password from the digital data file supplier (FIG. 6,Step 401). Then, the user downloads a digital data player 122 in asoftware form from the digital data server 110 through a communicationnetwork and sets the downloaded digital data player 122 in a personalcomputer 120.

Thereafter, the user transmits his ID number and password to the digitaldata server 110 through the personal computer 120 and communicationnetwork to download a desired digital data file from the digital dataserver 110 through the communication network. The digital data server110 identifies the user on the basis of the transmitted ID number andpassword and supplies the desired digital data file to the user inaccordance with the identification (FIGS. 4 and 6, Step 430). At thistime, the digital data server 110 encrypts the digital data file on thebasis of a predetermined encryption key and transmits the encrypteddigital data file to the personal computer 120.

The personal computer 120 stores the digital data file transmitted fromthe digital data server 110 on a hard disk 121 therein (FIG. 6, Step431). Then, upon receiving a reproduction request from the user, thepersonal computer 120 decrypts and reproduces the stored digital datafile through the digital data software player 122. As a result, the usercan listen to desired music through the personal computer 120.

On the other hand, in the case where the user intends to listen to musicin a digital data file form using a digital data playing device 130, thepersonal computer 120 has to transmit the desired digital data file tothe digital data playing device 130. In this case, if the digital datafile is powerfully encrypted and downloaded to the digital data playingdevice 130, a corresponding powerful decryption function must beperformed in the digital data playing device 130. Such powerfuldecryption would result in an increase in cost of the digital dataplaying device 130. Alternatively, when the digital data is downloadedunder the decrypted, or non-processed, condition, it is subject toillicit diversion during transmission, and subsequent uncontrolleddistribution. In order to overcome the above problems, the presentencryption apparatus encrypts and downloads the digital data file in thefollowing manner.

In the personal computer 120, an encrypted data file from the hard disk121 is decrypted by an encryption decryptor 123, temporarily stored in abuffer 124, and then transferred to an encryption/download unit 125(FIGS. 4 and 6, Steps: 470 and 480). Noticeably, different types of datafiles may be transferred to the encryption/download unit 125 along thesame path. At this time, it is determined whether a given data file mustbe protected during transmission because of a copyright. If the givendata file is determined to be copyrighted, then the buffer 124 isappropriately changed in size according to the size (capacity) of thegiven file (FIGS. 4 and 6, Steps: 490 and 500). To the contrary, in thecase where the given data file need not be protected because of acopyright, then it is directly downloaded to the digital data playingdevice 130 (FIGS. 4 and 6, Step: 510).

Changing the size of the buffer 124 appropriately as needed causes thedecrypted data not to be normally used even if it is hacked or illicitlydiverted while being stored in the buffer 124. As a result, the hackingor illicit diversion of the data file so protected does not frustratethe purposes of copyright laws, because the diverted file cannot be used

For example, assume that a digital data file to be protected incopyright has a three-minute capacity and it is normally reproducibleonly when being stored in the buffer 124 in the unit of two-secondamounts. In this case, the one-second amount-unit storage of the digitaldata file in the buffer 124 causes the digital data file not to benormally used even when it illegally flows. As a result, the copyrightof the digital data file can be protected. There may be various methodsfor identifying files to be protected in copyright. One such method isto identify files to be protected in copyright on the basis of extensionindexes.

In the case where the output digital data from the buffer 124 need notbe protected in copyright, it is downloaded directly in unencrypted formto the digital data playing device 130 through the encryption/downloadunit 125 (FIGS. 4 and 6, Step: 510). However, if the copyright of theoutput digital data from the buffer 124 must be protected, then the fileis weakly encrypted and downloaded to the digital data playing device130 by the encryption/download unit 125.

FIGS. 3A to 3E are views illustrating examples of file encryption inaccordance with the present invention. For weak encryption, a stronglyencrypted file as shown in FIG. 3A is partially decrypted on the basisof a predetermined encryption key and the remaining parts thereof areleft strongly encrypted. As a result, a file encrypted and downloaded bythe encryption/download unit 125 has striped, strongly encrypted areasas shown in FIG. 3B. Here shading denotes encrypted data and no shadingdenotes unencrypted data.

Alternatively, the file encrypted as shown in FIG. 3A may be totallydecrypted as shown in FIG. 3C and then more weakly encrypted/downloadedas shown in FIG. 3D on the basis of the predetermined encryption key.Alternatively, the unencrypted file shown in FIG. 3C may have onlyportions weakly encrypted as shown in FIG. 3E. Any of the encryptionschemes shown in FIG. 3B, 3D, or 3E reduce processing requirements forthe digital data playing device 130 relative to a file where all of thedata has been strongly encrypted. FIG. 6 also the encryption schemes ofFIGS. 3B, 3D, and 3E.

The digital data playing device 130 stores the digital data file fromthe personal computer 120, encrypted and downloaded in the above manner,in a memory 142 of a data storage medium 140 which may be in the form ofa removable card. If the user requests the digital data playing device130 to reproduce the digital data file stored in the memory 142, thenthe digital data playing device 130 reads the stored digital data filefrom the memory 142 and reproduces it through a decoder 132 therein. Atthis time, the digital data file read from the data storage medium 140has to be decrypted for the reproduction because it is in an encryptedform. For this reason, in the digital data playing device 130, amicrocomputer 131 decrypts the digital data file read from the datastorage medium 140 on the basis of the encryption key used in the aboveencryption procedure and outputs the decrypted digital data file to anoutput line through the decoder 132. Because of the weak encryptionshown, for example, in FIGS. 3B, 3D, and 3E, the decoder is keptlow-cost. As a result, the user can listen to desired music anywhereusing the digital data playing device 130 and the digital data file canbe prevented from illicit diversion and distribution while beingdownloaded to the digital data playing device 130.

In accordance with the present invention, the above encryption methodmay be implemented by one program on a recording medium. The encryptionprogram is configured to receive and store both digital data encryptedto a high level and an encryption key, decrypt the encrypted digitaldata according to the encryption key, store the decrypted digital datain a buffer in a predetermined unit of size, reencrypt output digitaldata from the buffer to a low level and download the reencrypted digitaldata to a digital data player or a data storage medium. This encryptionprogram is stored on a single recording medium for use in the digitaldata encryption of the present invention.

FIG. 4 is a flowchart illustrating a digital data file encryption methodin accordance with the present invention. A user of the presentinvention must have previously registered and have been assigned an IDnumber and password, as described earlier in the description of therelated art.

Thereafter, acccording to the present invention as shown in FIG. 4, uponreceiving a digital data file download request from a personal computerat Step 410, a server identifies the user at Step 420 to determinewhether the user is legitimate. At this time, the server identifies theuser on the basis of an ID code and password which are assigned from theserver to the user, as stated previously. If the user is identified tobe legitimate, the server downloads a desired digital data fileencrypted to a high level and an encryption key to the personal computerat Step 430. Upon downloading the desired digital data file from theserver, the personal computer determines at Step 440 whether a copyrightof the downloaded file must be protected. If the downloaded digital datafile need not be protected in copyright, then it is decrypted anddownloaded directly to a digital data player at Step 510. In this case,because no copyright problem is caused even when the downloaded file ishacked/diverted, there is no necessity for encrypting the downloadedfile to send it to the digital data player.

On the other hand, in the case where it is determined at the above Step440 that the copyright of the downloaded file must be protected, thepersonal computer first checks the capacity of the downloaded file atStep 450 and then sets an effective capacity of a buffer in accordancewith the checked result ate Step 460. Then, the personal computerdecrypts the downloaded file according to the encryption key at Step 470and stores the decrypted file in the buffer at Step 480 in such a mannerthat the decrypted file cannot be normally reproduced even when it ishacked in process of being downloaded to the digital data player. Thedigital data file stored in the buffer is encrypted to a low level atStep 490. This low level encryption does not require a separatemicroprocessor which is typically used for the high level encryption orpowerful encryption, thereby avoiding an increase in cost of theassociated playing device. Then, the digital data file encrypted to thelow level is downloaded to the digital data playing device at Step 500.

With reference to FIG. 5, there is shown in block form an alternativeembodiment of the digital data file encryption system in accordance withthe present invention. This second embodiment is substantially the samein construction as the first embodiment, with the exception that aplurality of digital data servers 110A-110C are provided. Because of theprovision of the plurality of digital data servers, the personalcomputer 120 performs the decryption operation, not using the singleencryption key as shown in FIG. 2, but using a plurality of encryptionkeys supplied respectively from the digital data servers. Then, thepersonal computer 120 reproduces the decrypted, or non-processed,digital data files or downloads them (with or without encryption) to thedigital data playing device 130 for storing in the storage medium 140 inthe same manner as stated previously.

FIG. 6 is a diagram illustrating the digital data file encryption methodshown in FIG. 4, and the flow of the data file among the components ofthe digital data file encryption apparatus shown in FIGS. 2 and 5. Afterthe user registers with a data file supplier (Step 400), and is assignedan ID number and password (Step 401), the user is authorized by thedigital data server to receive data files upon request. The request bythe user is fulfilled by the digital data server downloading the datafile (Step 430), the file having been encrypted to a high level by thepredetermined encryption key. The data file is received and stored inthe personal computer (Step 431), partially decrypted (Step 470) andthen stored by changing the size of the buffer (Step 480).

If the downloaded digital data file need not be protected in copyright,then it is decrypted and downloaded directly to a digital data player atStep 510. In this case, because no copyright problem is caused even whenthe downloaded file is hacked/diverted, there is no necessity forencrypting the downloaded file to send it to the digital data player. Onthe other hand, in the case where it is determined at the above step 440that the copyright of the downloaded file must be protected, the digitaldata file stored in the buffer is encrypted to a low level at step 490.As described above with reference to FIGS. 3B, 3D, and 3E, any of theencryption schemes 3B, 3D, and 3E may be used. This low level encryptiondoes not require a separate microprocessor which is typically used forthe high level encryption or powerful encryption, thereby avoiding anincrease in cost of the associated playing device. Then, the digitaldata file encrypted to the low level is downloaded to the digital dataplaying device at step 500.

According to the present invention, a digital data file downloaded froma single server is decrypted, stored in the buffer in a predeterminedunit of size, encrypted in a somewhat simple manner and downloaded tothe digital data player. Digital data files downloaded from a pluralityof servers are decrypted in individual decryption manners, encrypted inthe simple manner as mentioned above and downloaded to the digital dataplayer. Therefore, it is possible to reliably protect the copyright of agiven digital data file without increasing the cost of the digital dataplayer due to the decryption function.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method for copy protection, comprising: generating a partiallydecrypted data unit, the partially decrypted data unit including twoportions, one of the two portions is encrypted having a protection leveldifferent from the other of the two portions; and transferring thepartially decrypted data unit to a target device for further decryptingbased on information used in the step of generating the partiallydecrypted data unit; and further comprising the steps of: registering acomputer with a data server; transferring encrypted data from the dataserver to the computer the computer generating the partially decrypteddata unit in a first decryption unit of the computer; and using a seconddecryption unit of the target device to further decrypt the partiallydecrypted data unit based on the information used in the step ofgenerating the partially decrypted data unit.
 2. The method of claim 1,wherein the two portions having the different protection levels arespaced apart at a predetermined interval on the data unit.
 3. The methodof claim 2, further comprising the step of storing the partiallydecrypted data unit in a data storage medium or a digital data player.4. The method of claim 3, further comprising the step of reading thepartially decrypted stored data unit from the data storage medium or thedigital data player and reproducing the data unit at the request of auser.
 5. The method of claim 4, further comprising the step ofdecrypting the data unit based on a predetermined encryption key, andoutputting the decrypted data unit to an output line.
 6. The method ofclaim 2, further comprising the step of decrypting a remainder of thepartially decrypted data unit in the target device.
 7. The method ofclaim 1, wherein the data unit is partially decrypted based on apredetermined encryption key.
 8. The method of claim 2, wherein thepredetermined interval is a multiple or divisor of a buffer size.
 9. Themethod of claim 1, wherein the step of partially decrypting theencrypted data unit in the computer is performed at a plurality oflocations spaced apart at a predetermined interval on the partiallydecrypted data unit.
 10. The method of claim 1, further comprising thestep of storing the partially decrypted data unit in a data storagemedium or a digital data player.
 11. The method of claim 10, furthercomprising the step of reading the partially decrypted stored data unitfrom the data storage medium and reproducing the partially decrypteddata unit upon request of a user.
 12. The method of claim 11, furthercomprising the steps of: sending the partially decrypted digital dataunit to the digital data player; decrypting the reencrypted data unitbased on a predetermined encryption key; and outputting the decrypteddata unit to an output line of the digital data player.
 13. The methodof claim 1, wherein the data unit received by the target device ispartially decrypted based on a predetermined encryption key.
 14. Themethod of claim 13, further comprising the steps of: partiallydecrypting the encrypted data unit in the computer is performedindependently of operating the second decryption unit in the targetdevice.
 15. A method for copy protection, comprising: receiving a dataunit that has been encrypted based on a predetermined encryption key;identifying whether or not the received data unit needs to be protected;generating an encrypted data unit having a different encryption level ormethod from one used to encrypt the data unit, based on a result of theidentifying step; and transferring the generated encrypted data unithaving the different encryption level or method to a target device fordecrypting based on information used in the step of generating theencrypted data unit; and further comprising the steps of: registering acomputer with a data server; transferring the data unit from the dataserver to the computer; using a first decryption unit of the computerfor generating the encrypted data unit having the different encryptionlevel or method; and using a second decryption unit of the target devicefor decrypting the generated encrypted data unit having the differentencryption level or method based on the information used in the step ofgenerating the generated encrypted data unit.
 16. The method of claim15, wherein the generated encrypted data unit includes two portionshaving the different protection levels spaced apart at a predeterminedinterval on the data unit.
 17. The method of claim 16, furthercomprising the step of storing the generated encrypted data unit in adata storage medium or a digital data player.
 18. The method of claim17, further comprising the step of reading the generated encrypted dataunit from the data storage medium or the digital data player andreproducing the data unit at the request of a user.
 19. The method ofclaim 18, further comprising the step of decrypting the data unit basedon a predetermined encryption key, and outputting the decrypted dataunit to an output line.
 20. The method of claim 16, further comprisingthe step of decrypting the generated encrypted data unit in the targetdevice.
 21. The method of claim 16, wherein the step of generating theencrypted data unit is based on a predetermined encryption key.
 22. Themethod of claim 16, wherein the predetermined interval is a multiple ordivisor of a buffer size.
 23. A method for copy protection, comprisingthe steps of: enabling a registration mode for inputting a useridentification; receiving a data unit from a storage device based on theinputted user identification information, wherein the data unit has beenencrypted; generating an encrypted data unit having a differentencryption level or method from one used to encrypt the data unit; andtransferring the encrypted data unit having the different encryptionlevel or method to a target device for decrypting based on informationused in the step of generating the differently encrypted data unit; andfurther comprising the steps of: registering a computer with a dataserver; transferring the data unit from the data server to the computer;using a first decryption unit of the computer for generating theencrypted data unit having the different encryption level or method; andusing a second decryption unit of the target device for decrypting thegenerated encrypted data unit having the different encryption level ormethod based on the information used in the step of generating thegenerated encrypted data unit.
 24. The method of claim 23, wherein thestep of generating the encrypted data unit is performed at a pluralityof locations spaced apart at a predetermined interval on the generatedencrypted data unit.
 25. The method of claim 24, further comprising thestep of reading the generated encrypted data unit from the data storagemedium and reproducing the generated encrypted data unit upon request ofa user.
 26. The method of claim 25, further comprising the steps of:sending the generated encrypted data unit to the digital data player;decrypting the generated encrypted data unit based on a predeterminedencryption key; and outputting the decrypted data unit to an output lineof the digital data player.
 27. The method of claim 23, furthercomprising the step of storing the generated encrypted data unit in adata storage medium or a digital data player.
 28. The method of claim23, wherein the data unit received by the target device is encryptedbased on a predetermined encryption key.
 29. The method of claim 23,wherein the step of generating the encrypted data unit is performedindependently of decrypting the generated encryption unit in the targetdevice.
 30. A method for encrypting a digital data file, comprising:receiving a data file from a digital data server, the data file havingbeen encrypted in the digital data server based on a predeterminedencryption key; decrypting the data file using the predeterminedencryption key; identifying whether or not the received data file needsto be protected; reencrypting the decrypted data file on the basis ofthe identified result; and transferring the reencrypted data file to atarget device, wherein the reencrypted data file has a different levelof encryption as compared to that of the received data file that wasencrypted in the digital data server; and further comprising the stepsof: registering a computer with the digital data server; transferringthe data file from the digital data server to the computer; using afirst decryption unit of the computer for generating the reencrypteddata file having the different level of encryption; and using a seconddecryption unit of the target device to decrypt the reencrypted datafile having the different level of encryption based on the informationused in the step of reencrypting the decrypted data file.
 31. A methodfor managing content data, the method being performed by at least oneplaying device and comprising: receiving content data from an externalsource device, the content data being encrypted by a first encryptionmethod; controlling a duration time for the content data based on anidentifying operation, the content data being available on a buffer ofthe playing device for the duration time, and the identifying operationdetermining whether or not the duration time for the content data iscontrolled based on protection information associated with the contentdata; determining whether or not to change the first encryption methodbased on the protection information associated with the content data;and changing the first encryption method into a second encryption methodfor the content data when the determining step determines the firstencryption method is to be changed into the second encryption method,wherein the second encryption method is different from the firstencryption method and is selected among a plurality of predeterminedencrypting methods such that an external target device can decrypt thecontent data encrypted by the second encryption method.
 32. The methodof claim 31, wherein a size of the buffer is changed for the controllingthe duration time.
 33. The method of claim 31, wherein the changing stepcomprises: decrypting the content data being encrypted by the firstencryption method; and re-encrypting the decrypted content data by thesecond encryption method.
 34. The method of claim 33, furthercomprising: transmitting the content data being re-encrypted by thesecond encryption method to the external target device.
 35. An apparatusfor managing content data, the apparatus comprising: a buffer; acontroller programmed to: receive content data from an external sourcedevice, the content data being encrypted by a first encryption method;control a duration time for the content data based on an identifyingoperation, the content data being available in the buffer of the playingdevice for the duration time, and the identifying operation determiningwhether or not the duration time for the content data is controlledbased on protection information associated with the content data;determine whether or not to change the first encryption method based onthe protection information associated with the content data; and changethe first encryption method into a second encryption method for thecontent data when the first encryption method is determined to bechanged into the second encryption method, wherein the second encryptionmethod is different from the first encryption method and is selectedamong a plurality of predetermined encrypting method such that anexternal target device can decrypt the content data encrypted by thesecond encryption method.
 36. The apparatus of claim 35, wherein thecontroller is programmed to change a size of the buffer for controllingthe duration time.
 37. The apparatus of claim 35, further comprising: adecryptor programmed to decrypt the content data being encrypted by thefirst encryption method; and an encryption unit programmed to re-encryptthe decrypted content data by the second encryption method.
 38. Theapparatus of claim 37, wherein the controller is further programmed totransmit the content data being re-encrypted by the second encryptionmethod to the external target device.